Electrical machines are often used as prime movers connected to a driven equipment, such as a compressor or a pump. Electrical machines are also used as generators, drivingly connected to a prime mover, such as an internal combustion engine, for example a gas turbine or a reciprocating internal combustion engine.
Electrical machines are sometimes provided with a cooling fan mounted on the shaft thereof, for the generation of a flow of cooling fluid, such as typically a flow of cooling gas. In some known arrangements, electrical motors used as prime movers for turbo-compressors use the same gas processed by the turbo-compressor as cooling medium.
In a high-speed electrical machine there is typically a need to provide a significant flow of cooling gas to maintain the desired low temperature level in the rotor and stator of the electrical machine. In order to overcome the resistance to the cooling gas flow, the cooling fan supported on the shaft of an electrical machine is required to deliver a significant pressure on one end of the electrical machine. The pressure difference thus created along the rotor of the electrical machine results in axial force acting on the rotor of the electrical machine in the direction of the cooling flow, towards the end of the electrical machine opposite the cooling fan, i.e. towards the driven/driving machine connected to the electrical machine. The axial force generated by the cooling flow pressure may become very significant and requires a special thrust compensating device to be used.
Even if a powerful axial thrust compensation device, such as an axial bearing, is located in the machinery connected to the electrical rotating machine to compensate for its own axial force and is capable to compensate the additional force created in the connected electrical machine via the cooling gas flow, still the axial force of the cooling flow on the electrical machine rotor and shaft would make it difficult to select a flexible coupling to connect the shaft of the electrical machine with the shaft of the driven or driving machinery, which is often critical for high speed applications.
There is therefore a need for a different approach to the problem of compensating the axial thrust generated on the shaft of a rotating electrical machine generated by the flow of cooling gas provided by a cooling fan arranged on the electrical machine.